"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."

- Jonathan Swift

April 10, 2015

Edhazardia aedis

When two different parasites find themselves in a small host animal like a mosquito, there is only so much of the host to go around. So there is a pretty good chance that those co-occurring parasites are going to fight it out, and there's no guarantee that there will be a winner out of this conflict.

Edhazardia aedis is a microsporidian parasite that specialises on infecting Aedes aegypti - also known as the mosquito that can act as the main vector for a variety of viruses include those that causes degnue fever, yellow fever, and Chikungunya. Edhazardia aedis can spread through the mosquito population via two methods; (1) the parasite can proliferation throughout the mosquito's body until it ultimately overwhelms the host, which dies and dissolves into a cloud of infective spores, or (2) if an infected female mosquito survives the ordeal to adulthood and still manage to produce offspring, her mosquito babies will inherit E. aedis from her (gee, thanks a lot mum!).

But E. aedis can sometimes run into a competing species - Vavraia culicis. It is also a microsporidian, but unlike E. aedis, it is a generalist that can infect many different species of mosquitoes. It is also a mosquito-killer which has the same general modus operandi as E. aedis, where the parasite's spores are released when the host finally succumbs. This study found that mosquito larvae which have less access to food and are infected by both parasites tend to die earlier - and when the host dies, the spores are dispersed for both E. aedis and V. culicis - so everyone wins, right? Well not quite.

While host death does release the spores which allow them to infect more mosquito larvae, the parasites get more spores for their bucks by keeping their host alive for longer - so a host that ends up keeling over too early is not very cost effective. This applies to both E. aedis and V. culicis. Even before host death, the cost of co-infection starts manifesting itself. Regardless of whether the host dies sooner or later, both parasites produce less spores in co-infections. If E. aedis is sharing a host, it produces half as many spores as it would have if it had the host all to itself. But co-infection is even more costly for V. culicis, which manages to produce only a bit over a quarter of the spore it would have in single infections.

It is unknown how these two parasites duke it out in the mosquito, or why E. aedis has a competitive edge over V. culicis. Perhaps by being a specialist of A. aegypti, E. aedis has some sort of home ground advantage when it comes to getting the most out of its host. So it seems that some parasites just don't like sharing, and when it comes to living with others, sometimes it pays for a parasite to be a specialist.

3 comments:

I have no sense of scale, here. When you describe the host dissolving into a cloud of infectious spores, what's the elapsed time? Does a flying mosquito die by exploding into a spore cloud, or does a sickened mosquito land somewhere, stop moving, eventually die, then have its corpse dissolve into goo plus a cloud of spores?

And how tightly packed are mosquitoes that a cloud of spores is apt to infect some new mosquito passing through?

Hi, I don't have the answers to all of your questions. But here are some missing points :Edhazardia aedis spores infect Aedes larvae, which are aquatic. Spores then multiplicate in host (the larvae) and sometimes provocate death of the larvae and the release of the spores in the water. Thes spores can be easily dispersed by water flow until they reach another larvea that eat them.If the larvea does not die, then the pupae and finally the adult mosquito emerge, allowing a modified stage of the spore to be transmitted vertically (female to offsprings). The first stage of the spore, which is responsible for the death of larvae, don't continue to replicate in the adule mosquito. They all transform in the second form of the spore. That's why you won't see a mosquito explosion during a fly or a blood meal ;)I hope it can bring some lights to your questions and apologize for my bad english.

If the parasite kills the host while it is still a larva (in the water, because that's where they live), the mosquito larva dies and as it decomposes, the spores are released - hence the cloud of spores. A mosquito larva infected only with Edharardia aedis will release about 2 million spores into the water when it dies.

As I mentioned in the post, "if an infected female mosquito survives the ordeal to adulthood and still manage to produce offspring, her mosquito babies will inherit E. aedis from her"

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